1. Field of the Invention
The present invention relates to a repeating system for a digital radio wave transmission, and in particular, to the cancellation of transmitter-to-receiver interference at a repeater in a repeating system wherein carriers of the same frequency are used for both transmitter and receiver.
2. Description of the Prior Art
In a digital radio wave transmission system, a carrier, usually a microwave modulated by the digital baseband signal to be transmitted, is transmitted as a digital microwave radio signal. A repeater used in the digital microwave transmission system has at least one and, usually two repeating paths. Each repeating path comprises a pair of receiving and transmitting antennas. The digital microwave is received at the receiving antenna and is demodulated. The demodulated signal is applied to a decision circuit, which and regenerates the digital baseband signal. A carrier is modulated by the regenerated digital baseband signal at a transmitter, and the digital microwave signal is radiated from the transmitting antenna.
The receiving antenna of one repeating path and the transmitting antenna of the other path are mounted side by side on the front face of the repeater, while the transmitting antenna of the one repeating path and the receiving antenna of the other are arranged side by side on the opposite back face of the repeater.
In the repeater, signals transmitted from the two transmitting antennas may interfere with the desired signal received at each receiving antenna, due to the side-to-side and front-to-back antenna couplings.
In order to avoid such transmitter-to-receiver interference, a conventional repeating system uses a pair of different transmitting and receiving carrier frequencies in all hops over and over again. This system is called two frequency plan repeating system.
From the view point of frequency utilization efficiency, the two frequency repeating system is inferior to a single-frequency repeating system where the same frequency carrier pair is used for both transmitting and receiving carriers in common for all hops, although a cochannel dual polarization system has been used for various digital microwave radio systems.
In order to realize a single frequency repeating system, the transmitter-to-receiver interference must be eliminated. In this instance, the necessary bandwidth may be substantially reduced.